I think it has something to do with the density of the air. Wet air is denser, and more molecules come in contact with your skin than less dense, non-humid air, hence the perception of temperature is more intense.

I'm thinking of this based on ice baths. You can cool a bottle of wine down by placing it in a pitcher of ice, however, it will cool significantly faster if you put water in that pitcher with the ice... making an ice bath... since so many more molecules of "cold" come in contact with the bottle when the water is present versus when it's just dry air.

<warning: uninformed speculation follows>...Wet air is denser, and more molecules come in contact with your skin than less dense, non-humid air, hence the perception of temperature is more intense.

Actually, wet air is lighter than dry air. This is because a water molecule weighs less (18 g/mol), than either an N2 (28 g/mol) or O2 (32 g/mol). I found this surprising as well, as I thought of water as "heavy" compared to air, but it is water vapor that counts. The number of molecules in a gas pretty much depends on the conditions (heat, temperature, pressure) rather than on the composition of the gas, so it should be pretty much the same.

I'd agree with the heat capacity theory.

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I think it has something to do with the density of the air. Wet air is denser, and more molecules come in contact with your skin than less dense, non-humid air, hence the perception of temperature is more intense.

Moist air is less dense that dry air. A molecule of water has an atomic weight of 18. Nitrogen and oxygen molecules, with atomic weights of 28 and 32, make up the bulk of the atmosphere. PV=nRT (approximately), so two gases at the same pressure and temperature will have the same molecular density n/V but if they have different atomic weights, they will have different mass densities. Think of it this way: helium balloons rise.

Cold moist air feels colder than dry moist air (and hot moist air feels hotter than hot dry air) because moist air has a higher specific heat than does dry air. Diatomic nitrogen and oxygen molecules form the bulk of the atmosphere. Water is a polyatomic molecule that can rotate and translate in 3 dimensions. Moreover, the hydrogen atoms are not rigidly connected to the oxygen atom in a water molecule; the intermolecular bonds are a bit "springy". Diatomic oxygen and nitrogen molecules have a lot fewer degrees of freedom, and hence have a lower specific heat than does water vapor.

Thanks for the corrections, guys. I think it was mostly a poor word choice, and until this thread, I'd not previously heard the term "specific heat" or "heat capacity." I think I was on the right track with the idea that a water bath cools something faster than just ice, but lacked the terminology to convey that idea successfully.

I think of it as energy transfer. Our skin has receptors that measure the temperature of the skin. The skin is warmed by our blood, and cooled by outside air. If the cooling is rapid, then skin temperature will drop, and the skin receptors will report on the lower skin temperature.

"Skin receptor calling brain. Local report. It is bloody cold down here."

Since water is able to carry more heat energy than air (air is a good thermal insulator) then wet air can carry more heat energy away from the skin than dry air. More energy lost means temperature drop and the skin reports it is cold.

The vapor pressure of water at low temperatures means that the water concentration in air is <1%...

I agree that the specific heat capacity (Cp) and heat transfer are the factors that seem to be of importance here. The heat capacity of water vapor is about twice that of air (weighted average of N2 and O2). But that means that the overall difference of the Cp of dry air and wet air is about 1%, given its low vapor pressure...

To increase the mystery, I'll give some numbers:
The heat transfer coefficient of water vapor is 16 W/(m K)
The heat transfer coefficient of air is 24 W/(m K)

I looked up some equations for heat transfer... and I conclude that one factor that I didn't mention myself, and that I haven't seen here yet is the diffusion coefficient of gases. But, that doesn't seem to be the one either: the diffusion coefficient of water in air at 25 deg C is 25.6E-6 m2/s, while that of oxygen is 20.6E-6 m2/s. The difference is not very big...

Perhaps it's just psychology, not physics that makes that air feel colder?

It has to do with evaporation. The cold air has a partial pressure. When the moisture comes in contact with your skin it heats up and able to evaporate due to your warm body able to increase the vapor pressure. The evaporation will absorb heat taking more heat away from the body.

Another way to look at it, is the cold air can hold x water. Near the micro environment of you body it is warmer and can hold X+. There is a migration of water to you to balance out the chemical potential. There is also a heat conduction due to water being such a good heat absrober

If evaporation would have anything to do with it, I'd imagine that dry air feels colder: with less water in that air, your sweat will evaporate faster, therefore cooling you better. That's also why it feels so hot in a sauna when you throw some water on the hot stones (thereby increasing the moisture in the air: you reduce your sweating).

In wet air, the water already has evaporated. We're talking about water-gas, not droplets (fog and rain are crap, and you will feel miserable and cold, there's little science needed to explain why cold liquid water feels colder than air).

If evaporation would have anything to do with it, I'd imagine that dry air feels colder: with less water in that air, your sweat will evaporate faster, therefore cooling you better. That's also why it feels so hot in a sauna when you throw some water on the hot stones (thereby increasing the moisture in the air: you reduce your sweating).

yep, thats why hot muggy days feel like hot muggy days.

In wet air, the water already has evaporated. We're talking about water-gas, not droplets (fog and rain are crap, and you will feel miserable and cold, there's little science needed to explain why cold liquid water feels colder than air).

says it all.

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Captain, it may well be that psychology is more important than physics here! I cannot speak from experience because here in Durban, South Africa, we simply do not have the cold weather you speak of. But if I had to look for a physical reason I would look more to the human body than the air for the cause.

In summer it is the inability of the body's sweat to evaporate and cool the person that makes muggy hot weather feel worse than dry hot weather. Perhaps in winter if the air is dry it will cause a thin layer of dry skin to form on the body, making a thin insulating layer? In wet weather the skin will stay moist and be a better conductor of the body's heat out to the air, making it feel colder. This would make it the effect of the air's humidity on the body that impacts on how cold we feel, rather than the heat transfer properties of the air itself. But this is just conjecture.

it is not psychology as a warm object in cold wet air will cool faster than a warm object in bone dry air (I have done experiments which show this even though we were looking at a different parameter).

humans would percieve this as 'feeling colder'.

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At what conditions did you do those measurements (temperature, and air humidity differences)? And how must faster was the object cooling down?

Somehow I think that katmar's reply makes sense too... the cold air will prevent sweat from evaporating properly. This can make the tiny hairs on your skin sticky, thus preventing them from standing up. That could reduce the thickness of the thin stagnant layer of air that insulates your body.

At 95%RH, 120 deg C and 5 bar the water constitutes about 38% of the gas. This is surely enough for the specific heat difference to become significant. I don't think this is comparable with CaptainPanic's conditions.

Water vapour does have a higher heat capacity than air; but that's not the whole story. It also has a higher conductivity.
Imersed in wet air (a better conductor) your surface attains a temperature nearer that of its surroundings than it would surrounded by dry air (a poorer conductor). If, as in the OP those surroundigs are colder than you, you will feel colder in wet air.

At 95%RH, 120 deg C and 5 bar the water constitutes about 38% of the gas. This is surely enough for the specific heat difference to become significant. I don't think this is comparable with CaptainPanic's conditions.

it is comparable. the effect is proportionally greater.

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